Flatten Nested List Iterator solution and explanation

class NestedIterator:
    def __init__(self, nestedList):
        self.flatList = []
        self.index = -1
        self.flatten(nestedList)

    def flatten(self, nestedList):
        for element in nestedList:
            if element.isInteger():
                self.flatList.append(element.getInteger())
            else:
                self.flatten(element.getList())

    def next(self):
        self.index += 1
        return self.flatList[self.index]

    def hasNext(self):
        return self.index + 1 < len(self.flatList)

public class NestedIterator implements Iterator<Integer> {
    private List<Integer> flatList;
    private int index;

    public NestedIterator(List<NestedInteger> nestedList) {
        flatList = new ArrayList<>();
        index = 0;
        flatten(nestedList);
    }

    private void flatten(List<NestedInteger> nestedList) {
        for (NestedInteger element : nestedList) {
            if (element.isInteger()) {
                flatList.add(element.getInteger());
            } else {
                flatten(element.getList());
            }
        }
    }

    public Integer next() {
        return flatList.get(index++);
    }

    public boolean hasNext() {
        return index < flatList.size();
    }
}

class NestedIterator {
    constructor(nestedList) {
        this.stack = [];
        this._flatten(nestedList);
    }
    
    _flatten(nestedList) {
        for (let i = nestedList.length - 1; i >= 0; i--) {
            this.stack.push(nestedList[i]);
        }
    }
    
    hasNext() {
        while (this.stack.length) {
            let top = this.stack.pop();
            if (top.isInteger()) {
                this.stack.push(top);
                return true;
            }
            this._flatten(top.getList());
        }
        return false;
    }
    
    next() {
        return this.stack.pop().getInteger();
    }
}

public class NestedIterator {
    private Stack<NestedInteger> stack = new Stack<NestedInteger>();

    public NestedIterator(IList<NestedInteger> nestedList) {
        Flatten(nestedList);
    }

    private void Flatten(IList<NestedInteger> nestedList) {
        for (int i = nestedList.Count - 1; i >= 0; i--) {
            stack.Push(nestedList[i]);
        }
    }

    public bool HasNext() {
        while (stack.Count > 0) {
            NestedInteger top = stack.Pop();
            if (top.IsInteger()) {
                stack.Push(top);
                return true;
            }
            Flatten(top.GetList());
        }
        return false;
    }

    public int Next() {
        return stack.Pop().GetInteger();
    }
}

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *   public:
 *     // Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     bool isInteger() const;
 *
 *     // Return the single integer that this NestedInteger holds, if it holds a single integer
 *     // The result is undefined if this NestedInteger holds a nested list
 *     int getInteger() const;
 *
 *     // Return the nested list that this NestedInteger holds, if it holds a nested list
 *     // The result is undefined if this NestedInteger holds a single integer
 *     const vector<NestedInteger> &getList() const;
 * };
 */

class NestedIterator {
public:
    NestedIterator(vector<NestedInteger>& nestedList) {
        auto dfs = [&](this auto&& dfs, vector<NestedInteger>& ls) -> void {
            for (auto& x : ls) {
                if (x.isInteger()) {
                    nums.push_back(x.getInteger());
                } else {
                    dfs(x.getList());
                }
            }
        };
        dfs(nestedList);
    }

    int next() {
        return nums[++i];
    }

    bool hasNext() {
        return i + 1 < nums.size();
    }

private:
    vector<int> nums;
    int i = -1;
};

/**
 * Your NestedIterator object will be instantiated and called as such:
 * NestedIterator i(nestedList);
 * while (i.hasNext()) cout << i.next();
 */

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * class NestedInteger {
 *     If value is provided, then it holds a single integer
 *     Otherwise it holds an empty nested list
 *     constructor(value?: number) {
 *         ...
 *     };
 *
 *     Return true if this NestedInteger holds a single integer, rather than a nested list.
 *     isInteger(): boolean {
 *         ...
 *     };
 *
 *     Return the single integer that this NestedInteger holds, if it holds a single integer
 *     Return null if this NestedInteger holds a nested list
 *     getInteger(): number | null {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a single integer equal to value.
 *     setInteger(value: number) {
 *         ...
 *     };
 *
 *     Set this NestedInteger to hold a nested list and adds a nested integer elem to it.
 *     add(elem: NestedInteger) {
 *         ...
 *     };
 *
 *     Return the nested list that this NestedInteger holds,
 *     or an empty list if this NestedInteger holds a single integer
 *     getList(): NestedInteger[] {
 *         ...
 *     };
 * };
 */

class NestedIterator {
    private nums: number[] = [];
    private i = -1;
    constructor(nestedList: NestedInteger[]) {
        const dfs = (ls: NestedInteger[]) => {
            for (const x of ls) {
                if (x.isInteger()) {
                    this.nums.push(x.getInteger());
                } else {
                    dfs(x.getList());
                }
            }
        };
        dfs(nestedList);
    }

    hasNext(): boolean {
        return this.i + 1 < this.nums.length;
    }

    next(): number {
        return this.nums[++this.i];
    }
}

/**
 * Your ParkingSystem object will be instantiated and called as such:
 * var obj = new NestedIterator(nestedList)
 * var a: number[] = []
 * while (obj.hasNext()) a.push(obj.next());
 */

/**
 * // This is the interface that allows for creating nested lists.
 * // You should not implement it, or speculate about its implementation
 * type NestedInteger struct {
 * }
 *
 * // Return true if this NestedInteger holds a single integer, rather than a nested list.
 * func (this NestedInteger) IsInteger() bool {}
 *
 * // Return the single integer that this NestedInteger holds, if it holds a single integer
 * // The result is undefined if this NestedInteger holds a nested list
 * // So before calling this method, you should have a check
 * func (this NestedInteger) GetInteger() int {}
 *
 * // Set this NestedInteger to hold a single integer.
 * func (n *NestedInteger) SetInteger(value int) {}
 *
 * // Set this NestedInteger to hold a nested list and adds a nested integer to it.
 * func (this *NestedInteger) Add(elem NestedInteger) {}
 *
 * // Return the nested list that this NestedInteger holds, if it holds a nested list
 * // The list length is zero if this NestedInteger holds a single integer
 * // You can access NestedInteger's List element directly if you want to modify it
 * func (this NestedInteger) GetList() []*NestedInteger {}
 */

type NestedIterator struct {
	nums []int
	i    int
}

func Constructor(nestedList []*NestedInteger) *NestedIterator {
	var dfs func([]*NestedInteger)
	nums := []int{}
	i := -1
	dfs = func(ls []*NestedInteger) {
		for _, x := range ls {
			if x.IsInteger() {
				nums = append(nums, x.GetInteger())
			} else {
				dfs(x.GetList())
			}
		}
	}
	dfs(nestedList)
	return &NestedIterator{nums, i}
}

func (this *NestedIterator) Next() int {
	this.i++
	return this.nums[this.i]
}

func (this *NestedIterator) HasNext() bool {
	return this.i+1 < len(this.nums)
}

// #[derive(Debug, PartialEq, Eq)]
// pub enum NestedInteger {
//   Int(i32),
//   List(Vec<NestedInteger>)
// }
struct NestedIterator {
    nums: Vec<i32>,
    i: usize,
}

/**
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
impl NestedIterator {
    fn new(nested_list: Vec<NestedInteger>) -> Self {
        let mut nums = Vec::new();
        Self::dfs(&nested_list, &mut nums);
        NestedIterator { nums, i: 0 }
    }

    fn next(&mut self) -> i32 {
        let result = self.nums[self.i];
        self.i += 1;
        result
    }

    fn has_next(&self) -> bool {
        self.i < self.nums.len()
    }

    fn dfs(nested_list: &Vec<NestedInteger>, nums: &mut Vec<i32>) {
        for ni in nested_list {
            match ni {
                NestedInteger::Int(x) => nums.push(*x),
                NestedInteger::List(list) => Self::dfs(list, nums),
            }
        }
    }
}